Fatigue of aluminum

endurance failure

Like other metals, aluminum and alloys destroyed by repeated loads at stress levels significantly lower, than for static loads. This type of fracture, which is the formation of cracks under the influence of alternating voltage, It is known as fatigue failure.

These alternating voltages in practice may be caused by alternating loads, vibration or repeated temperature changes. Direction, wherein the fatigue crack propagates, It is always perpendicular to the line voltage feedback, cause cracks. When a crack grows, voltage in the remainder of the cross-section increases and accordingly increases and crack growth rate.

Finally, stage reached, when the remaining part of the cross section can no longer withstand the applied loads and the specimen fails. Fatigue cracks are very difficult to notice, since in contrast to the test specimen at break no visible thinning of the specimen at the place of destruction.

Cycle fatigue loading

To describe the state of stress, leading to fatigue, the following four main parameters apply:
1) minimum cycle voltage, p_min;
2) maximum cycle voltage , p_Max;
3) average cycle voltage, p_m = (p_min + p_Max)/2;
4) amplitude of the cycle voltage, p_a = (p_min – p_Max)/2.

Voltage cycle is fully defined, if you know any two of these four parameter.

The influence of the average cycle stress on fatigue strength is very significant: the imposition of additional tensile stresses on the symmetric cycle reduces the fatigue strength, imposition of compressive stress enhances ee.

stress concentrators

Fatigue resistance is usually greatly reduced by the presence of local stress concentrators, such as acute groove, sharp change even thickness profile or rough machining marks and scratches.

Particular influence on fatigue strength, these surface defects and stress concentrators have, if they are located transversely to the direction of stress application. They may have a much greater influence on the fatigue strength, than those differences, which may be from different alloys or various treatments. Therefore always key considerations for the fatigue strength of structural members are of regular shape and surface quality.

Weller curve for aluminum

The usual procedure is to test the fatigue test, a certain number of samples at the different levels of voltage amplitude. When applying the test results on a graph voltage-number of cycles to failure is obtained fatigue curve (Figure 1). It is also often referred to as curve Weller and S-N diagram.

In steels fatigue curve has a flat portion, defining endurance strength. At stresses below this fatigue limit, the material will never fail.. Most of the aluminum alloys, as well as other non-ferrous metal, do not have such a limit (the only exception can be aluminum-magnesium 5xxx series alloys), and the curve is sloping indefinitely. Therefore, for most aluminum alloys are always a result of cyclic loading will be destruction, although the range of values ​​greater durability slope of the curves becomes small.

Fig. 1 – Difference in Fatigue Behaviour between Mild Steel and Aluminium Alloys [1]

Fatigue of aluminum

To denote voltage, at which destruction of aluminum alloys (and most other non-ferrous alloys) occurs, the term is used fatigue strength at a predetermined durability, which is denoted by σ_N. The term fatigue strength determines the voltage amplitude, in which the destruction occurs after a specified number of cycles. The term also always corresponds to the fatigue limit of infinite durability.

of tensile strength data, yield strength and fatigue strength of some aluminum alloys in the various states shown in comparison in Figure 2.

 

Fig. 2 – Fatigue, Proof and Ultimate Tensile Strengths
of some Wrought Aluminium Alloys [1]

Source:

1. TALAT 1501